The present invention relates to the mixing of polymer gel agents and water to form a well treatment fluid, such as a fracturing or acidizing gel, and more particularly, but not by way of limitation, to a method and apparatus for continuously mixing such gels on a real time basis.
High viscosity aqueous fluids, such as fracturing gels, acidizing gels, and high density completion fluids, are commonly used in the oil industry in treating subterranean wells. These gels are normally made using dry polymer additives or agents which are mixed with water or other aqueous fluids at the job site. The mixing procedures which have been used have inherent problems. For example, the earliest "batch" mixing procedures involved mixing bags of the polymer in tanks at the job site. This created problems such as uneven and inaccurate mixing, lumping of the powder into insoluble "gel balls" or "fish eyes" which obstructed the flow of the gel, chemical dust hazards, etc.
A known method of solving the lumping, gel ball problem is to delay hydration long enough for the individual polymer particles to disperse and become surrounded by water so that no dry particles are trapped inside a gelled coating to form a gel ball. This delay is achieved by coating the polymer with material such as borate salts, glyoxal, non-lumping HEC, sulfosuccinate, metallic soaps, surfactants, or other materials of opposite surface charge to the polymer.
Another known way to improve the efficiency of polymer addition to water and derive the maximum yield from the polymer is to prepare a stabilized polymer slurry ("SPS"), also referred to as a liquid gel concentrate ("LGC"). The liquid gel concentrate is premixed and then later added to the water. In U.S. Pat. No. 4,336,145 to Briscoe, assigned to the assignee of the present invention, a liquid gel concentrate is disclosed comprising water, the polymer or polymers, and an inhibitor having a property of reversibly reacting with the hydratable polymer in a manner wherein the rate of hydration of the polymer is retarded. Upon a change in the pH condition of the concentrate such as by dilution and/or the addition of a buffering agent (pH changing chemical) to the concentrate, upon increasing the temperature of the concentrate, or upon a change of other selected condition of the concentrate, the inhibition reaction is reversed, and the polymer or polymers hydrate to yield the desired viscosified fluid. This reversal of the inhibition of the hydration of the gelling agent in the concentrate may be carried out directly in the concentrate or later when the concentrate is combined with additional water. The aqueous-based liquid gel concentrate of Briscoe has worked well at eliminating gel balls and is still in routine use in the industry. However, aqueous concentrates can suspend only a limited quantity of polymer due to the physical swelling and viscosification that occurs in a water-based medium. Typically, about 0.8 pounds of polymer can be suspended per gallon of the concentrate.
By using a hydrocarbon carrier fluid, rather than water, higher quantities of solids can be suspended. For example, up to about five pounds of polymer may be suspended in a gallon of diesel fuel carrier. Such a liquid gel concentrate is disclosed in U.S. Pat. No. 4,722,646 to Harms and Norman, assigned to the assignee of the present invention. Such hydrocarbon-based liquid gel concentrates work well but require a suspension agent such as an organophylic clay or certain polyacrylate agents. The hydrocarbon-based liquid gel concentrate is later mixed with water in a manner similar to that for aqueous-based liquid gel concentrates to yield a viscosified fluid, but hydrocarbon-based concentrates have the advantage of holding more polymer.
An additional problem with prior methods using liquid gel concentrates occurs in offshore and remote locations. The service vehicles utilized to supply the offshore and remote locations have a limited storage capacity and often must return to their source to replenish their supply of concentrate before they are able to complete large jobs or do additional jobs, particularly when the liquid gel concentrate is water-based. Therefore, it would be desirable to be able to continuously mix a well treatment gel during the actual treatment of the subterranean formation from dry ingredients. For example, such an on-line system could satisfy the fluid flow requirements for large hydraulic fracturing jobs during the actual fracturing of the subterranean formation by continuously mixing the fracturing gel.
One method and apparatus for continuously mixing a fracturing gel is disclosed in U.S. Pat. No. 4,828,034 to Constien et al., in which a fracturing fluid slurry concentrate is mixed through a static mixer device 3 on a real time basis and the slurry is flowed through baffled tanks 4, 7 in a first-in first-out flow pattern to produce a fully hydrated fracturing fluid during the actual fracturing operation. This process utilizes a hydrophobic solvent which is characterized by a hydrocarbon such as diesel, as in the hydrocarbon-based liquid gel concentrates described above.
Recently, however, there have been problems with hydrocarbon-based liquid gel concentrates. Some well operators object to the presence of hydrocarbon fluids, such as diesel, even though the hydrocarbon represents a relatively small amount of the total fracturing gel once mixed with water. Also, there are environmental problems associated with the clean-up and disposal of both hydrocarbon-based concentrates and well treatment gels containing hydrocarbons; as well as with the clean-up of the tanks, piping, and other handling equipment which have been contaminated by the hydrocarbon-based gel. These hydrocarbon-related problems apply to the process of Constien et al.
Accordingly, there is a need for a process to produce a well treatment gel in which relatively higher amounts of polymer per unit volume can be utilized while eliminating the environmental problems and objections related to hydrocarbon-based concentrates. There is also a need for apparatus and method to produce a well treatment gel substantially continuously during the well treatment operation to overcome the storage capacity problems discussed above.
U.S. patent application Ser. No. 07/693,995, to Harms et al., which is incorporated herein by reference thereto for purposes of disclosure, assigned to the assignee of the present invention, discloses method and apparatus for substantially continuously producing a fracturing gel, without the use of hydrocarbons or suspension agents, by feeding the dry polymer into an axial flow mixer which uses a high mixing energy to wet the polymer during its initial contact with water. After initial mixing, additional water may be added to the mixer to increase the volume of water-polymer slurry produced thereby. In Harms et al., a predetermined quantity of hydratable polymer in a substantially particulate form is provided to a polymer or solids inlet of a water spraying mixer. A stream of water is supplied to a water inlet of the mixer and the water and polymer are mixed in the mixer to form a water-polymer mix prior to discharge from the mixer. The mixer is preferably mounted adjacent to the upper portion of a mixing or primary tank and an agitator may be provided in the mixing tank to further agitate and stir the slurry. The slurry may be transferred from the mixing tank to a holding or secondary tank after which it is discharged to the fracturing process. A high shear device may be disposed in the holding tank. A pump may be used for transferring the slurry from the mixing tank to the holding tank.
Although Harms et al. disclose an on-line mixing system which may be used with untreated and uncoated polymers, in practice there are problems with the Harms et al. mixing system. For example, the powder splatters inside the mixer, sticks to the walls of the mixer, and builds up, eventually choking flow through the mixer. The sequential opening of the water orifices in sets of six orifices inadequately wets the powder at low flow rates, and allows unwetted powder to pass. Another problem is created by the entrainment of air in the fluid mixed in the mixer which impairs the ability of the pump to adequately pump the mixture from the mixer. Another problem is the creation of additional entrained air in the fluid in the holding tank by the discharge of the pump into the holding tank. The entrained air compels the use of deaerating chemicals with the system. Another problem is the lack of a controlled flow path and therefore the hydration time in the holding tank, i.e., the hydrating slurry can create unpredictable flow channels through the tank which cause non-uniform residence times of portions of the slurry in the tank. Another problem is the large lag time (5-10 minutes) involved in changing the viscosity of the gel discharged from the holding tank, i.e., the only way to alter the viscosity of the gel is to change the powder/water ratio at the mixer and therefore the fluid of "altered" viscosity must displace all of the fluid and gel between the mixer and the outlet of the holding tank before the viscosity at the outlet of the holding tank is altered.
Therefore, there is a need for an apparatus and method for hydrating a particulated polymer which will fully wet the dry polymer powder while reducing splattering and gel buildup inside the mixer; which will eliminate voids and openings in the water spray pattern through which unwetted powder can pass; which will reduce the entrainment of air in the polymer water mixture; which will eliminate the need for deaerating chemicals; which will provide for instantaneous adjustment of the viscosity of the produced gel; and which will do so continuously, i.e., which will wet the powder and produce the gel on-line as demanded by the gel user, thereby reducing the need for hydration tanks and other gel contacting containers at the job site.